[GreenKeys] Milliamp meter shunting...

[drlegendre .]

Ken,

As you might have concluded, a meter's scale markings may not indicate the
actual "rating" of the meter. You can have a meter marked for "RF Watts
X1000" which by no means directly measures 1000s of RF watts.. but rather,
as a current driven by a voltage across some external low-impedance shunt /
detector mechanism.

But mainly, it's important to understand that +all+ analog meters are
+current+ meters, in some form. This should be obvious, in that it requires
a given current through the meter winding to produce a certain amount of
deflection. Analog mechanical "volt" meters are really just high-impedance
ammeters, with the best requiring as little as 50uA to drive full-scale.

So you can see how such a very sensitive movement might be viewed as
voltage-driven, being that it will still give an acceptable reading even if
the source impedance is relatively high and the resultant current quite
low.



On Sat, May 6, 2017 at 1:37 PM, Ralph Mowery <rmowery28146 at earthlink.net>
wrote:

> It would be hard to tell without knowing the model of the meter.
>
>
>
> Some of the meters that require shunts will have on the front of the scale
> near the bottom in very small words something along the line of meter full
> scale 1 ma or FS=1 ma. Or whatever the meter movement actually is.
>
>
>
>
>
>
>
> *From:* GreenKeys [mailto:greenkeys-bounces at mailman.qth.net] *On Behalf
> Of *Kenneth Gartland
> *Sent:* Saturday, May 06, 2017 1:49 PM
> *To:* greenkeys
> *Subject:* [GreenKeys] Milliamp meter shunting...
>
>
>
> Hi guys...
>
>
>
> I have a couple of old Simpson ma meters rated
>
> at 100 ma...
>
>
>
> I went to use them to test a known 40ma cct.
>
> the other day, and both pegged at over the scale limit...
>
>
>
> Do these older meters require some type of "Shunt"
>
> across the terminals to get an accurate reading?
>
>
>
> I know the newer meters can be directly connected in
>
> series with the circuit load...
>
>
>
> I used to know this kind of stuff, but too many brain farts
>
> these days to remember!
>
>
>
> Tks for help on this issue!
>
>
>
> Cheers!
>
>
>
> Ken
>
> VE3-HMQ
>
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